A. Field of the Invention
The present invention relates to electronic storage and reproduction of design images, and in particular to preparation of printing plates for and direct output of complex color designs.
B. Description of the Related Art
In commercial printing applications, color images are usually printed in separate, successive stages of ink or dye application. Each layer of color is applied directly atop the previous stage, generally by a printing device having a series of single-color inking stations that each make sequential, registered contact with the substrate.
The number of color stages required for complete image rendition depends on the "color model" chosen as the basis for representation. Although the CYMK model, consisting of blue (cyan), magenta, yellow, and black components is frequently employed in commercial applications involving paper substrates, a wide variety of models is employed for textile and surface designs. Multicolored images are separated into a set of monochrome representations in accordance with the selected color model, each separation corresponding to the contribution of a particular color to the overall image.
The process of generating suitable separations can be quite complex, particuarly in cases involving mass reproduction of artistic images. Graphic designers typically work with a large palette of colors, necessitating highly accurate and versatile separation techniques to retain these colors throughout the printing process. Furthermore, printing materials--particularly non-paper substrates--tend to react differently to various dyes or inks (hereafter referred to generically as "color carriers"); for example, the wicking properties of fabric frequently vary with the applied color carrier, resulting in different degrees of spreading (or "bleeding") beyond the borders of application. The order of color-carrier application can also be important, since the amount of wicking may depend, for example, on whether dye is applied to dry fabric or to a portion that has already been dyed and fixed. Finally, the extent and character of interaction among color carriers as they are applied can depend strongly on the particular color combination.
Until recently, graphic artists have created works using traditional materials, which were reproduced photographically and processed using optical techniques into separations. Developments in computer-aided design ("CAD") technology, however, have considerably broadened the degree to which graphic artists can render complex color designs electronically. CAD-generated images are digitally stored as discrete picture elements, or "pixels," with each pixel corresponding to an identified image point. The entire set of pixels representing an image is termed a "pixelmap" of the image. While simple monochrome image pixels can consist of single computer-memory bits, each of which indicates whether the corresponding image point is light or dark, color image pixels require additional specification information (e.g., values for hue, color value, and saturation, or an index for a specific dye formulation). As used herein, the term "pixelmap" refers to raster pattern of pixels and/or bits. A multicolored pixelmap can be subjected to various algorithmic operations, now well-characterized in the art, to resolve the image into color separations.
While such color-separation algorithms accurately implement a color model, they do not capture and reflect the myriad variations in final appearance that can result from interactions between applied color carriers and the substrate and among themselves. Failure to render the separations in a manner that accommodates characteristics of the printing process results in visually perceptible, and therefore unacceptable, deviations from the artist's original conception.
Unfortunately, the complexity of the numerous possible interactions makes their management quite difficult. A graphic designer's work may be nearly as complex as a continuous-tone photograph, with considerable density and hue variations occurring over small image areas and, frequently, also including regions of fine detail. Global calibrations that affect all instances of a particular color ignore the effect of adjacent colors, while adjustments on a pixel-by-pixel basis that account for the identities of neighboring pixels are impractical. Furthermore, some deviation in the final printed image from the artist's original design may be necessary to accommodate mechanical limitations of printing equipment in achieving perfect registration.